In an image forming apparatus such as an electrophotographic apparatus or electrostatic recording apparatus, a printed image is generally formed in accordance with the following process.
An electrostatic latent image is first formed on a photosensitive member by conducting image exposure. The electrostatic latent image is then developed with a developer (toner) to form a visible image (toner image). The development of the electrostatic latent image is conducted by a method that a toner stored in a development unit is supplied on to the photosensitive member using a means such as a developing roll or developing belt. The toner image on the photosensitive member is transferred to any of various transfer media such as paper and OHP sheets as needed. The toner image on the transfer medium is fixed on to the transfer medium by a means such as heating, pressing or solvent vapor. On the other hand, the toner remaining on the photosensitive member without being transferred is removed by a cleaning means. A printed image is formed by such a series of operations. As the developer, is generally used a toner for development of electrostatic images (hereinafter may be referred to as “toner” merely) comprising, as a main component, colored resin particles containing a binder resin and a colorant.
In recent years, there has been strong need of forming color images in image forming apparatus using an electrophotographic system, such as copying machines, facsimiles and printers. In color printing, high resolution is required because printing of high-definition images such as photographs is also conducted. Therefore, color toners capable of forming a high-resolution and high-definition image are required.
In addition, reduction of printing cost is also required. Therefore, a toner is required not to lower print quality even when printing is conducted on a large number of sheets of paper, i.e., to have excellent printing durability. Further, the toner has recently been required to lower neither image quality nor printing durability even under a high-temperature and high-humidity environment.
As a method for coping with the above-described various requirements, it has heretofore been known to adding inorganic fine particles and/or organic fine particles having a particle diameter smaller than a particle diameter of colored resin particles to the colored resin particles so as to cause such fine particles to adhere to the surfaces of the colored resin particles. In other words, it is attempted to improve resolution and printing durability by externally adding the inorganic fine particles and/or organic fine particles to the colored resin particles which become a main component of a toner.
Japanese Patent Application Laid-Open No. 3-100661 discloses a toner obtained by externally adding fine silica particles having an average particle diameter of from 5 nm or more to less than 20 nm and fine titanium dioxide particles having an average particle diameter of from 20 nm or more to 40 nm or less in combination to spherical toner particles having an average particle diameter of from 2 to 6 μm (colored resin particles).
However, the toner obtained by externally adding the fine silica particles and fine titanium dioxide particles to the colored resin particles is improved in transferability, developability and cleaning ability, but lowered in flowability and easy to cause fog due to embedding of the external additives in the colored resin particles when a printing durability test is conducted on a large number of sheets of paper.
Japanese Patent Application Laid-Open No. 54-28630 proposes a toner obtained by externally adding silane-modified clay to toner particles (colored resin particles). This document shows clay modified with a silane coupling agent such as aminosilane or mercaptosilane.
Japanese Patent Application Laid-Open No. 5-158276 discloses a nonmagnetic one-component developer obtained by externally adding spherical fine powder made of glass having an average particle diameter of 1 to 50 μm to colored fine particles (colored resin particles). This document proposes externally adding colloidal silica or clay together with the spherical fine powder made of glass. Specifically, the document discloses, as the clay, kaolin clay (calcined clay), the surface of which has been subjected to a hydrophobilizing treatment with a silane coupling agent or silicone oil. The kaolin clay is a layered compound. However, when it is calcined, crystal water is lost to lose its layer structure.
The toner obtained by externally adding the silane-modified clay or the calcined clay subjected to the hydrophobilizing treatment to the colored resin particles is improved in printing durability to some extent. However, the improvement is insufficient to meet the requirement level in recent years, and so its charge level is lowered, and fog is easy to occur when a printing durability test is conducted under a high-temperature and high-humidity environment.